Constructive parent-child conflict interactions that teach children to problem-solve and negotiate can enhance children's social adjustment. This paper identifies constructive and destructive qualities of mother-child conflict and explores whether child temperament moderated associations with changes in externalizing problems over time. One hundred and ninety mothers and their 5- to 7-year-old children participated in a laboratory conflict discussion rated on aspects of maternal sensitivity, child mood, and mothers' and children's contemptuous and planning comments. Mothers also reported on children's negative reactivity and their externalizing behavior problems concurrently and one year later. Structural equation modeling revealed that constructive conflict quality was related to decreases in children's externalizing problems, but only among children low in negative reactivity.